The majority of current semiconductor devices are made from semiconductor materials based on silicon (Si), gallium arsenide (GaAs), and indium phosphide (InP). Compared to such electronic and optoelectronic devices, GaN devices have many advantages. The major intrinsic advantages that GaN has are summarised in Table 1:
TABLE 1Band GapBFOM(eV)/(powerMaximumSemi-Mobility μwavelengthtransistorTemperatureconductor(cm2/Vs)(nm)merit)(C.)Si1300 1.1/11271.0300GaAs50001.4/8869.6300GaN15003.4/36024.6700BFOM: Baliga's figure of merit for power transistor performanceA shorter wavelength corresponds to a higher DVD/CD capacity.
From Table 1, it can be seen that GaN has the highest band gap (3.4 eV) among the given semiconductors. Thus, it is called a wide band gap semiconductor. Consequently, electronic devices made of GaN operate at much higher power than Si and GaAs and InP devices. Green, blue, and ultraviolet (UV) and white light emitting diodes (LEDs) can be made from GaN wafers.
For semiconductor lasers, GaN lasers have a relatively short wavelength. If such lasers are used for optical data storage, the shorter wavelength may lead to a higher capacity. GaAs lasers are used for the manufacture of CD-ROMs with a capacity of about 670 MB/disk. AlGaInP lasers (also based on GaAs) are used for the latest DVD players with a capacity of about 4.7 GB/disk. GaN lasers in the next-generation DVD players may have a capacity of 26 GB/disk.
GaN devices are made from GaN wafers that are typically multiple GaN-related epitaxial layers deposited on a sapphire substrate. The sapphire substrate is usually two inches in diameter and acts as the growth template for the epitaxial layers. Due to lattice mismatch between GaN-related materials (epitaxial films) and sapphire, defects are generated in the epitaxial layers. Such defects cause serious problems for GaN lasers and transistors and, to a lesser extent, for GaN LEDs.
There are two major methods of growing epitaxial wafers: molecular beam epitaxy (MBE), and metal organic chemical vapour deposition (MOCVD). Both are widely used.
Conventional LED fabrication processes usually include the major steps: photolithography, etching, dielectric film deposition, metallization, bond pad formation, wafer inspection/testing, wafer thinning, wafer dicing, chip bonding to packages, wire bonding and reliability testing.
Once the processes for making LEDs are completed at the full wafer scale, it is then necessary to break the wafer into individual LED chips or dice. For GaN wafers grown on sapphire substrates, this “dicing” operation is a major problem as sapphire is very hard. The sapphire first has to be thinned uniformly from about 400 microns to about 100 microns. The thinned wafer is then diced by diamond scriber, sawed by a diamond saw or by laser grooving, followed by scribing with diamond scribers. Scribing of the sapphire may be by use of an ultra violet (“UV”) laser, but care must be taken to ensure the laser does not damage the GaN device. Such processes limit throughput, cause yield problems and consume expensive diamond scribers/saws.
Known LED chips grown on sapphire substrates require two wire bonds on top of the chip. This is necessary because sapphire is an electrical insulator and current conduction through the 100-micron thickness is not possible. Since each wire bond pad takes about 10-15% of the wafer area, the second wire bond reduces the number of chips per wafer by about 10-15% as compared to single-wire bond LEDs grown on conducting substrates. Almost all non-GaN LEDs are grown on conducting substrates and use one wire bond. For packaging companies, two wire bonding reduces packaging yield, requires modification of one-wire bonding processes, reduces the useful area of the chip, and complicates the wire bonding process.
Sapphire is not a good thermal conductor. For example, its thermal conductivity at 300 K (room temperature) is 40 W/Km. This is much smaller than copper's thermal conductivity of 380 W/Km. If the LED chip is bonded to its package at the sapphire interface, the heat generated in the active region of the device must flow through 3 to 4 microns of GaN and 100 microns of sapphire to reach the package/heat sink. For GaN LEDs on sapphire, the active region where light is generated is about 3 to 4 micron from the sapphire substrate. As a consequence, the chip will run hot affecting both performance and reliability.
In general, the external quantum efficiency of GaN LEDs is less than the internal quantum efficiency. If no special treatment is carried out on the chip to extract more light, the external quantum efficiency is a few percent, while the internal quantum efficiency can be as high as 99% (I. Schnitzer and E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Schere, Applied Physics Letters, Volume 63, page 2174, 18 Oct. 1993). This large discrepancy between the two quantum efficiencies is also true to other LEDs. Its origin is due to the light extraction efficiency of most conventional LEDs being limited by the total internal reflection of the generated light in the active region of the LED, which occurs at the semiconductor-air interface. This is due to the large difference in the refractive index between the semiconductor and air.
For GaN devices, the critical angle to enable the light generated in the active region to be able to escape is about 23°. Because the light emission from the active region of a LED is directionally isotropic, and the light can only escape from the chip if the angle of incidence to the chip wall (often the front surface of the LED chip) is less than the critical angle, a small fraction of light generated in the active region of the LED can escape to the surrounding environment (e.g. air). The escaping light is generally in a cone of light. FIG. 1 (not to scale) illustrates this escape cone concept. Therefore, for a conventional LED, the external quantum efficiency is limited to a few percent.
It is known that surface texturing can increase the light extraction efficiency significantly (e.g., I. Schnitzer and E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Schere, Applied Physics Letters, Volume 63, page 2174, 18 Oct. 1993), and it has been used in the fabrication of LEDs such as, for example, AlGaInP-based LEDs. In order to increase light-extraction efficiency from LEDs, it is very important that the photons generated within the LEDs experience multiple opportunities to find the escape cone or the surface is so modified that the generated light falls into new escape cones, as shown in the illustration of FIG. 2, which is a figure from Journal of Applied Physics, Volume 93, page 9383, 2003.
It has been suggested to improve the light output of an InGaN-based LED by using a microroughened p-GaN surface (i.e. the normal top or front surface), with metal clusters being used as a wet etching mask (Journal of Applied Physics 93, page 9383, 2003). The light-output efficiency of an LED structure with a microroughened surface was significantly increased compared to that of a conventional LED structure. For an LED with a p-GaN top surface that was microroughened, the angular randomization of photons can be achieved by surface scattering from the microroughened top surface of the LED. Thus, the microroughened surface structure can improve the probability of photons escaping to outside the LED, resulting in an increase in the light output power of the LED.
The technique of surface texturing, however, has only been applied to the front surface of the LED. The technique has difficulties in device fabrication, especially for GaN LEDs, where the layers above the active region are quite thin (about 300 nm), and etching of GaN is difficult. To texture the surface, patterns of depths of a few hundred nanometers are often generated by dry etching or wet etching on the surface. This poses considerable risk of possible damage to the active region, and thus may cause a significant deterioration in the performance of the device.